Spectrometers are used to measure the properties of light for a variety of applications including environmental or chemical analysis.
An example of a prior art sensor is the LaserSense™ system, described in Reference [13] listed below, which is incorporated herein by reference. Reference [1] listed below, which is incorporated herein by reference, describes the use of such a system, and in particular a LaserScan™ system for detection of solid residues on a probed surface. Another example is the LaserWarn™ system, described in Reference [14] listed below, which is incorporated herein by reference.
These systems contain a laser source of tunable wavelength or wavenumber and a photodetector. Light of only one wavenumber is transmitted at a given time and the single-wavelength light is directed in a collimated beam toward a probed surface. Light from the vicinity of the probed surface is then collected and detected by a photodetector in the system. The laser light for the LaserScan™ system, referenced in Reference [1], can be tuned over a 600 cm−1 span, for example, between 1430 and 830 cm−1 and transmits short pulses of relatively low average power (0.5-10 mW). The pulses are produced at a rate of 200 kHz. This sensor transmits an output beam of approximately 2 mm×4 mm size and a beam divergence of 5 mrad.
An active spectrometer with laser illumination also can be constructed from a combination of a laser source with tunable output wavenumber and a separate photodetector or a separate infrared imager having an array of photodetectors. Examples of such spectrometers are described in References [2], [3] and [4], which are incorporated herein by reference. Tunable laser sources, which typically have one or more quantum cascade lasers, are available such as the LaserTune™ system, described in Reference [15] listed below, which is incorporated herein by reference. Other such systems include the MIRcat™ system, described in Reference [16] listed below, which is incorporated herein by reference, and the OmniLux™ system described in Reference [17] listed below, which is incorporated herein by reference. All of these laser sources output laser light having only one wavenumber or wavelength at a time, so that only one wavenumber or wavelength is transmitted at any one time. The photodetector can be thermoelectrically (TE) cooled to a temperature as low as 195K or cryogenically cooled to liquid nitrogen (LN2) temperatures (˜77-80K).
A disadvantage of the prior laser sources is that the output power can vary by almost one order of magnitude as the wavelength is tuned. Another disadvantage is that it can take one second or more to obtain a spectrum of 600 cm=1 spectral span. These prior spectrometers have external-cavity lasers that mechanically move an optical element such as a grating or a mirror in the laser cavity to accomplish the wavelength tuning. Also, these prior spectrometers do not include a built-in means to spatially move the beam of output light and instead obtain spatially distinguishable spectra over a large area by illuminating the entire area with a fixed beam and then using an infrared imager that has an array of multiple photodetectors to provide the needed spatial discrimination. Thus, since the illuminating light is spread out over the large area, the illumination power at any given portion of the probed surface is low and the acceptable standoff distance is small (typically 0.5 to 2 meters).
Another prior tunable laser source, described in Reference [5] listed below, which is incorporated herein by reference, has an array of distributed feedback (DFB) lasers for which each DFB laser is designed to emit light of a slightly different wavelength. The desired output wavelength for the array is obtained by switching on only one DFB laser at a time. A given laser of the array is switched on by increasing its drive current to a value exceeding the lasing-threshold current of that laser. The threshold current and slope efficiency of output power vs drive current of the different lasers in an array are different. Thus, the output power of the array can change as the wavelength of the light emitted is changed, which is an undesirable characteristic for a spectrometer. The outputs from the various lasers of an array can be combined into a single optical beam, using a cascade to two gratings, so that the same spot on a probed surface continues to be illuminated as the wavelength of the light is changed, as described in References [6] and [7] listed below, which are incorporated herein by reference.
Prior spectrometers having a laser array activate only one laser and wavelength or wave number at a time and produce a time-varying wavelength scan, as described in References [5] and [6] listed below, which are incorporated herein by reference, because in the prior art, the photodetectors do not have any means to distinguish the wavelength of the light it detects.